The goal of this application is to develop and validate spectral-domain imaging technology at visible wavelengths to collect and reconstruct 2-dimensional topographic and 3-dimensional tomographic images of the oxidation state of mitochondrial cytochrome c and cytochrome oxidase, as well as the absolute hemoglobin concentration and saturation and the concentration of extrinsic chromophores. The technology will have the spatial and depth precision sufficient to resolve the columnar and laminar structure of the mammalian neocortex, and have the temporal precision to resolve events at the timescale of the neuronal circuits. It could be applied minimally invasively in mice and neonatal rats where the skull is sufficiently thin to be transparent, on rats using a standard thinned skull preparation, on awake non-human primates using a window preparation and on patients under going neurosurgery. We anticipate that this technology will be able to address fundamental issues in brain physiology such as mitochondrial oxygenation during neuronal activation, the magnitude of oxygen gradients in the brain, mechanisms of activation-flow coupling, the role of the blood flow increases during activation, the apparent shift from oxidative to glycolytic metabolism during focal activation and the origin of the elusive "initial dip" observed in some but not all functional activation studies. The ability to image the columnar and laminar structure of the cortex with the temporal precision of the neuronal circuits has the potential to unravel the manner in which the cortex processes information. Furthermore, the technology has an application during neurosurgery to locate the boundaries of a tumor or to precisely locate a seizure focus during a lesionectomy. We have four specific aims (i) Construct a raster scanning visible spectral domain imaging system, (ii) Develop high resolution spectral-domain topographic and tomographic reconstruction algorithms based on a rapid Monte Carlo forward model, (iii) Develop methods to image total blood volume and tissue hematocrit and test the hypothesis that changes in the tissue hematocrit are insensitive to changes in the draining venules and so provide more localized vascular images of activation (iv) Develop techniques to image the cortex on the timescale of the neuronal circuits and test the hypothesis that a component of the fast optical signal is metabolic in origin and can be imaged with the mitochondrial signals.